Curing agent for silicone rubber compounds

Abstract

What are presented and described are a curing agent for silicone rubber compounds comprising a compound having the general structural formula R.sup.1.sub.mSi(R).sub.4-m, a process for preparation thereof and the use of this curing agent for hardening a silicone rubber compound. The invention further relates to a composition comprising the curing agent for silicone rubber compounds and to the use of such a composition as sealant, adhesive or coating composition.

Claims

1. A curing agent for silicone rubber compounds, comprising a compound having the general structural formula R.sup.1.sub.mSi(R).sub.4-m where each R.sup.1 is independently an optionally substituted straight-chain or branched C1- to C16-alkyl group, an optionally substituted straight-chain or branched C2- to C16-alkenyl group, or an optionally substituted C4- to C14-aryl group, m is an integer from 0 to 2, wherein (a) R is a hydroxycarboxylic ester radical having the general structural formula (II): ##STR00012## where R.sup.2 is an optionally substituted straight-chain or branched C1- to C16-alkyl group, a C4- to C14-cycloalkyl group, or a C5- to C15-aralkyl group, with the proviso that R.sup.2 is not ethyl, benzyl, or phenethyl, or oligomers or polymers of the curing agent, or (b) the general structural formula R.sup.1.sub.mSi(R).sub.4-m is ##STR00013## or oligomers or polymers of the curing agent, wherein R.sup.2 is an optionally substituted straight-chain or branched C1- to C16-alkyl group, a C4- to C14-cycloalkyl group, a C5- to C15-aralkyl group, or a C4- to C14-aryl group, or (c) R is a hydroxycarboxylic ester radical having the general structural formula (I): ##STR00014## where R.sup.2 is an optionally substituted straight-chain or branched C1- to C16-alkyl group, a C4- to C14-cycloalkyl group, a C5- to C15-aralkyl group, or a C4- to C14-aryl group, with the proviso that R.sup.2 is not ethyl, benzyl or phenyl, wherein Z is (CR.sup.3R.sup.4).sub.n in which n is an integer from 2 to 10, or an optionally substituted saturated or partly unsaturated cyclic ring system having 4 to 14 carbon atoms, each R.sup.3 and R.sup.4 is independently H or an optionally substituted straight-chain or branched C1- to C16 alkyl group or a C4- to C14-aryl group, and n is an integer from 2 to 10, or oligomers or polymers of the curing agent.

2. The curing agent according to claim 1, wherein n is an integer from 2 to 5.

3. The curing agent according to claim 1, wherein each R.sup.3 and R.sup.4 is independently H or an optionally substituted straight-chain or branched C1- to C12-alkyl group.

4. The curing agent according to claim 1, wherein each R.sup.3 and R.sup.4 is independently selected from the group consisting of H, methyl, ethyl, propyl, butyl, n-butyl, sec-butyl, isobutyl and tert-butyl.

5. The curing agent according to claim 1, wherein R is a hydroxycarboxylic ester radical having the general structural formula (I), and wherein R.sup.2 in general structural formula (I) is an optionally substituted straight-chain or branched C1- to C12-alkyl group, a C4- to C10-cycloalkyl group, a C5- to C11-aralkyl group, or a C4- to C10-aryl group, with the proviso that R.sup.2 is not ethyl, benzyl or phenyl.

6. The curing agent according to claim 5, wherein R.sup.2 is selected from the group consisting of methyl, propyl, isopropyl, butyl, n-butyl, sec-butyl, isobutyl, tert-butyl, pentyl, n-pentyl, sec-pentyl, 3-pentyl, 2-methylbutyl, isopentyl, 3-methylbut-2-yl, 2-methylbut-2-yl, neopentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, ethylhexyl, 2-ethylhexyl, naphthyl and cyclohexyl.

7. The curing agent according to claim 1, wherein R is a hydroxycarboxylic ester radical having the general structural formula (II), and wherein R.sup.2 in general structural formula (II) is an optionally substituted straight-chain or branched C1- to C12-alkyl group, a C4- to C10-cycloalkyl group or a C5- to C11-aralkyl group, with the proviso that R.sup.2 is not ethyl, benzyl, or phenethyl.

8. The curing agent according to claim 7, wherein R.sup.2 is selected from the group consisting of methyl, propyl, isopropyl, butyl, n-butyl, sec-butyl, isobutyl, tert-butyl, pentyl, n-pentyl, sec-pentyl, 3-pentyl, 2-methylbutyl, isopentyl, 3-methylbut-2-yl, 2-methylbut-2-yl, neopentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, ethylhexyl, 2-ethylhexyl, naphthyl and cyclohexyl.

9. The curing agent according to claim 8, wherein R.sup.2 is selected from the group consisting of methyl, isopropyl, n-butyl and 2-ethylhexyl.

10. The curing agent according to claim 1, wherein each R.sup.1 is independently an optionally substituted straight-chain or branched C1- to C12-alkyl group, an optionally substituted straight-chain or branched C2- to C12-alkenyl group, or an optionally substituted C4- to C10-aryl group.

11. The curing agent according to claim 10, wherein each R.sup.1 is independently methyl, ethyl, propyl, vinyl, phenyl or allyl.

12. The curing agent according to claim 1, wherein the general structural formula R.sup.1.sub.mSi(R).sub.4-m is ##STR00015## wherein R.sup.2 is an optionally substituted straight-chain or branched C1- to C12-alkyl group, a C4- to C10-cycloalkyl group, a C5- to C10-aryl group, or a C5- to C11-aralkyl group.

13. The curing agent according to claim 12, wherein R.sup.2 is selected from the group consisting of methyl, ethyl, propyl, isopropyl, butyl, n-butyl, sec-butyl, isobutyl, tert-butyl, pentyl, n-pentyl, sec-pentyl, 3-pentyl, 2-methylbutyl, isopentyl, 3-methylbut-2-yl, 2-methylbut-2-yl, neopentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, ethylhexyl, 2-ethylhexyl, benzyl, phenyl, naphthyl and cyclohexyl.

14. The curing agent according to claim 1, wherein the general structural formula R.sup.1.sub.mSi(R).sub.4-m is ##STR00016## or oligomers or polymers of the curing agent.

15. A curing agent for silicone rubber compounds, comprising at least one compound selected from the group of consisting of tris(2-ethylhexylsalicylato)ethyl silane, tris(2-ethylhexylsalicylato)methyl silane, tris(2-ethylhexylsalicylato)phenyl silane, tris(2-ethylhexylsalicylato)propyl silane, tris(2-ethylhexyl-salicylato)vinyl silane, tris(ethylhexylsalicylato)ethyl silane, tris(ethylhexylsalicylato)methyl-silane, tris(ethylhexylsalicylato)phenyl silane, tris(ethylhexylsalicylato)propyl silane, tris-(ethylhexylsalicylato)vinyl silane, tris(isopropylsalicylato)ethyl silane, tris(isopropylsalicylato)-phenyl silane, tris(isopropylsalicylato)propyl silane, tris(isopropylsalicylato)vinyl silane, tris-(methylsalicylato)ethyl silane, tris(methylsalicylato)methyl silane, tris(methylsalicylato)phenyl-silane, tris(methylsalicylato)vinyl silane, tetra(2-ethylhexylsalicylato)silane, tetra(ethylhexyl-salicylato)silane, tetra(isopropylsalicylato) silane, and tetra(methylsalicylato)silane, or oligomers or polymers of the curing agent, or mixtures thereof.

16. A curing agent for silicone rubber compounds, comprising a compound having the structural formula: ##STR00017## where R.sup.1 is vinyl and R.sup.2 is ethylhexyl, or oligomers or polymers of the curing agent.

17. The curing agent according to claim 1, comprising at least two different compounds from said general structural formula R.sup.1.sub.mSi(R).sub.4-m.

18. The curing agent of claim 1, obtainable by mixing together at least two different compounds from said general structural formula R.sup.1.sub.mSi(R).sub.4-m.

19. A process for preparing the curing agent according to claim 1, comprising reacting a compound having the general formula R.sup.1.sub.mSiX.sub.4-m with (4m) equivalents of at least one hydroxycarboxylic ester, where each R.sup.1 is independently an optionally substituted straight-chain or branched C1- to C16-alkyl group, an optionally substituted straight-chain or branched C2- to C16-alkenyl group, or an optionally substituted C4- to C14-aryl group, m is an integer from 0 to 2, X is selected from the group consisting of an alkoxy radical having at least one carbon atom, and a halogen atom, and wherein said at least one hydroxycarboxylic ester is selected from the group consisting of: (a) a hydroxycarboxylic ester having the general structural formula (II): ##STR00018## where R.sup.2 is an optionally substituted straight-chain or branched C1- to C16-alkyl group, a C4- to C14-cycloalkyl group, or a C5- to C15-aralkyl group, with the proviso that R.sup.2 is not ethyl, benzyl, or phenethyl, or oligomers or polymers of the curing agent, or (b) a hydroxycarboxylic ester that reacts with said compound having the general formula R.sup.1.sub.mSiX.sub.4-m to generate a compound of the following formula ##STR00019## wherein R.sup.2 is an optionally substituted straight-chain or branched C1-to C16-alkyl group, a C4- to C14-cycloalkyl group, a C5- to C15-aralkyl group, or a C4- to C14-aryl group, or (c) a hydroxycarboxylic ester having the general structural formula (III) ##STR00020## where R.sup.2 is an optionally substituted straight-chain or branched C1- to C16-alkyl group, a C4- to C14-cycloalkyl group, a C5- to C15-aralkyl group, or a C4- to C14-aryl group, with the proviso that R.sup.2 is not ethyl, benzyl or phenyl, wherein Z is (CR.sup.3R.sup.4).sub.n in which n is an integer from 2 to 10, or a saturated or partly unsaturated cyclic ring system having 4 to 14 carbon atoms, each R.sup.3 and R.sup.4 is independently H or an optionally substituted straight-chain or branched C1- to C16-alkyl group or a C4- to C14-aryl group, and n is an integer from 2 to 10.

20. The process according to claim 19, wherein each R.sup.1 is independently an optionally substituted straight-chain or branched C1- to C12-alkyl group, or an optionally substituted straight-chain or branched C2- to C12-alkenyl group, or an optionally substituted C4- to C10-aryl group.

21. The process according to claim 19, wherein each R.sup.3 and R.sup.4 is independently H or an optionally substituted straight-chain or branched C1- to C12-alkyl group.

22. The process according to claim 19, wherein R.sup.2 is an optionally substituted straight-chain or branched C1- to C12-alkyl group, a C4- to C10-cyclohexyl group, a C5- to C11-aralkyl group, or a C4- to C10-aryl group, with the proviso that R.sup.2 is not ethyl, benzyl or phenyl.

23. The process according to claim 19, wherein each R.sup.1 is independently methyl, ethyl, propyl, vinyl, phenyl or allyl and/or the hydroxycarboxylic ester is characterized by one or more of the following features: R.sup.2 is an optionally substituted straight-chain or branched C1- to C8-alkyl group, a C4- to C10-cycloalkyl group, or a C4- to C10-aryl group, with the proviso that R.sup.2 is not ethyl or phenyl; and R.sup.3 and R.sup.4 is independently H or an optionally substituted straight-chain or branched C1- to C12-alkyl group.

24. The curing agent according to claim 1, wherein R is a hydroxycarboxylic ester radical having the general structural formula (II).

25. A method for hardening a silicone rubber compound, said method comprising mixing a silicone rubber compound with the curing agent according to claim 1.

26. A composition comprising a curing agent according to claim 1, and an organosilicon compound.

27. The composition according to claim 26, wherein the organosilicon compound is an alpha,omega-dihydroxyl-terminated polyorganosiloxane compound.

28. A composition obtainable by mixing at least one curing agent of claim 1 with at least one bonding agent (adhesion promoter) and at least one further component selected from the group consisting of thixotropic agent, organosilicon compound, filler, colorant, plasticizer, wetting agent and catalyst.

29. The composition according to claim 24, wherein said composition is suitable as a sealant, adhesive, potting compound and/or a coating composition.

30. A method for curing a silicone rubber compound, wherein said method comprises mixing a silicone rubber compound with a curing agent comprising a compound having the general structural formula R.sup.1.sub.mSi(R).sub.4-m where each R.sup.1 is independently an optionally substituted straight-chain or branched C1- to C16-alkyl group, an optionally substituted straight-chain or branched C2- to C16-alkenyl group, or an optionally substituted C4- to C14-aryl group, m is an integer from 0 to 2, R is a hydroxycarboxylic ester radical having the general structural formula (I): ##STR00021## where R.sup.2 is an optionally substituted straight-chain or branched C1- to C16-alkyl group, a C4- to C14-cycloalkyl group, a C5- to C15-aralkyl group, or a C4- to C14-aryl group, wherein Z is (A) an optionally substituted aromatic group having 4 to 14 carbon atoms, where, when m=2, m=1 or m=0, the C6-aryl group is not ##STR00022## and, when m=2 or m=0 and the C6-aryl group is ##STR00023## R.sup.2 is not phenyl; and, when the C6-aryl group is ##STR00024## R.sup.2 is not ethyl or H (B) (CR.sup.3R.sup.4).sub.n, wherein n is an integer from 2 to 10 or (C) an optionally substituted saturated or partly unsaturated cyclic ring system having 4 to 14 carbon atoms, each R.sup.3 and R.sup.4 is independently H or an optionally substituted straight-chain or branched C1- to C16 alkyl group or a C4- to C14-aryl group, and n is an integer from 2 to 10, or oligomers or polymers of the curing agent.

31. The method according to claim 30, wherein said curing agent comprises a compound selected from the group of consisting of tris(2-ethylhexyl-salicylato)ethylsilane, tris(2-ethylhexylsalicylato)methylsilane, tris(2-ethylhexylsalicylato)-phenyl silane, tris(2-ethylhexylsalicylato)propylsilane, tris(2-ethylhexylsalicylato)vinylsilane, tris(ethylhexylsalicylato)ethylsilane, tris(ethylhexylsalicylato)methyl silane, tris(ethylhexyl-salicylato)phenylsilane, tris(ethylhexylsalicylato)propylsilane, tris(ethylhexylsalicylato)vinyl-silane, tris(ethylsalicylato)ethylsilane, tris(ethylsalicylato)methylsilane, tris(ethylsalicylato)-phenyl silane, tris(ethylsalicylato)propylsilane, tris(ethylsalicylato)vinyl silane, tris(isopropyl-salicylato)ethylsilane, tris(isopropylsalicylato)phenylsilane, tris(isopropylsalicylato)propylsilane, tris(isopropylsalicylato)vinylsilane, tris(methylsalicylato)ethyl silane, tris(methylsalicylato)-methyl silane, tris(methylsalicylato)phenylsilane, tris(methylsalicylato)vinylsilane, tetra(2-ethylhexylsalicylato)silane, tetra(ethylhexylsalicylato)silane, tetra(ethylsalicylato)silane, tetra(isopropylsalicylato)silane, and tetra(methylsalicylato)silane, or oligomers or polymers of the curing agent, or mixtures thereof.

Description

EXAMPLES

Syntheses of the Curing Agents

Example 1

Synthesis of tris(ethylsalicylato)methylsilane (M(ES)3)

(1) In a 4 l four-neck flask with precision glass stirrer, reflux condenser, dropping funnel and temperature sensor, under a nitrogen atmosphere, 604.20 g (534.69 ml; 3.64 mol) of ethyl salicylate (ES), and 382.50 g (523.97 ml; 3.78 mol) of triethylamine (TEA) are dissolved in 1920 g (2206.90 ml; 20.84 mol) of toluene. 179.40 g (141.26 ml; 1.20 mol) of methyltrichlorosilane (MTS) are metered in via dropping funnel at room temperature while stirring in such a way that the temperature of the reaction mixture always remains below 30 C. On completion of addition, the reaction mixture is stirred at room temperature and under a nitrogen atmosphere for 3 hours. The precipitate formed is filtered off under inert conditions and the filtercake is washed three times with 165 g each time (total of 570 ml; 5.43 mol) of toluene. Ten drops of cyclohexylamine are added to the filtrate, and it is subsequently freed of the solvent under reduced pressure. In this way, 629.1 g (1.16 mol), corresponding to a yield of 97% (based on methyltrichlorosilane), of the desired tris(ethylsalicylato)methylsilane product (M(ES)3) with a Cl.sup. content of 20 ppm are obtained.

Example 2

Synthesis of tris(ethylsalicylato)propylsilane (P(ES)3)

(2) In a 4 l four-neck flask with precision glass stirrer, reflux condenser, dropping funnel and temperature sensor, under a nitrogen atmosphere, 604.20 g (534.69 ml; 3.64 mol) of ethyl salicylate (ES), and 382.50 g (523.97 ml; 3.78 mol) of triethylamine (TEA) are dissolved in 1920 g (2206.90 ml; 20.84 mol) of toluene. 213.04 g (179.78 ml; 1.20 mol) of propyltrichlorosilane (PTS) are metered in via dropping funnel at room temperature while stirring in such a way that the temperature of the reaction mixture always remains below 30 C. On completion of addition, the reaction mixture is stirred at room temperature and under a nitrogen atmosphere for 3 hours. The precipitate formed is filtered off under inert conditions and the filtercake is washed three times with 165 g each time (total of 570 ml; 5.43 mol) of toluene. Ten drops of cyclohexylamine are added to the filtrate, and it is subsequently freed of the solvent under reduced pressure. In this way, 652.1 g (1.15 mol), corresponding to a yield of 96% (based on propyltrichlorosilane), of the desired tris(ethylsalicylato)propylsilane product (P(ES)3) with a Cl.sup. content of 20 ppm are obtained.

Example 3

Synthesis of tris(ethylsalicylato)vinylsilane (V(ES)3)

(3) In a 4 l four-neck flask with precision glass stirrer, reflux condenser, dropping funnel and temperature sensor, under a nitrogen atmosphere, 604.20 g (534.69 ml; 3.64 mol) of ethyl salicylate (ES), and 382.50 g (523.97 ml; 3.78 mol) of triethylamine (TEA) are dissolved in 1920 g (2206.90 ml; 20.84 mol) of toluene. 193.79 g (152.59 ml; 1.20 mol) of vinyltrichlorosilane (VTS) are metered in via dropping funnel at room temperature while stirring in such a way that the temperature of the reaction mixture always remains below 30 C. On completion of addition, the reaction mixture is stirred at room temperature and under a nitrogen atmosphere for 3 hours. The precipitate formed is filtered off under inert conditions and the filtercake is washed three times with 165 g each time (total of 570 ml; 5.43 mol) of toluene. Ten drops of cyclohexylamine are added to the filtrate, and it is subsequently freed of the solvent under reduced pressure. In this way, 627.72 g (1.14 mol), corresponding to a yield of 95% (based on vinyltrichlorosilane) of the desired tris(ethylsalicylato)vinylsilane product (V(ES)3) with a Cl.sup. content of 21 ppm are obtained.

(4) In an analogous manner, further silanes with hydroxycarboxylic esters as leaving groups were prepared using the corresponding reactants, for example 2-ethylhexyl salicylate.

(5) Silicone Rubber Mixtures (Sealant Formulations)

(6) In addition, silicone rubber mixtures (sealant formulations) comprising curing agents of the invention were examined.

(7) The product properties of skin time, tack-free time, through-curing and elongation at break of the silicone rubber mixtures (sealant formulations) were determined after deployment of the sealants by customary methods. Unless stated otherwise, the measurements were conducted at 23 C. and 50% air humidity.

(8) The skin time indicates the time at which, after deployment of the sealant on the surface of a sample strand, a complete layer of solidified material (skin) has been observed. The tack-free time indicates the time at which the surface of a sample strand is no longer tacky. For the determination of complete curing, the sealant is applied to a glass plate with a height of 9 mm and the duration of through-curing down to the glass plate is measured. Elongation at break was determined to DIN EN ISO 8339:2005-09.

Example 4

Sealant Formulation A

(9) In the case of sealant formulations A, B and C, silicone rubber mixtures were produced in each case according to the following formulation:

(10) TABLE-US-00001 580 g alpha,omega-dihydroxyl-terminated polydimethylsiloxane having viscosity of 80 000 cSt 260 g polydimethylsiloxane having viscosity of 100 cSt 1 g aminoalkyltrialkoxysilane 91 g untreated fumed silica 0.2 g catalyst: alkyltin carboxylate 9 g adhesion promoter: aminoalkyltrialkoxysilane

(11) Curing agent A was added:

(12) TABLE-US-00002 56 g P(ES)3-tris(ethylsalicylato)propylsilane

(13) The resulting sealant, after deployment under air, has a skin time of 2 min a tack-free time of 15 min a transparent appearance a Shore A hardness of 24

Sealant Formulation B

(14) Curing agent B was added:

(15) TABLE-US-00003 56 g P(SEHE)3-tris(2-ethylhexylsalicylato)propylsilane

(16) The resulting sealant, after deployment under air, has a skin time of 2 min a tack-free time of 29 min a transparent appearance a Shore A hardness of 16

Example 4 (Prior Art; cf. EP2030976 A1, Example 7)

(17) Sealant Formulation C with Lactate Curing Agent Mixture

(18) As curing agent C, a mixture of the following formulation was added:

(19) TABLE-US-00004 20 g vinyltris(ethyllactato)silane 20 g methyltris(ethyllactato)silane

(20) The resulting sealant, after deployment under air, has a skin time of 12 min a tack-free time of 60 min a transparent appearance a Shore A hardness of 22

Example 5

(21) A silicone rubber mixture of the following composition was produced by mixing the components under reduced pressure:

(22) TABLE-US-00005 580 g alpha,omega-dihydroxyl-terminated polydimethylsiloxane having viscosity of 80 000 cSt 264 g polydimethylsiloxane having viscosity of 100 cSt 56 g curing agent: P(ES)3 1 g aminoalkyltrialkoxysilane 91 g untreated fumed silica 0.2 g catalyst: alkyltin carboxylate 9 g adhesion promoter: dimethylaminopropyltrimethoxysilane

(23) The resulting sealant was transparent and colourless and had a skin time of 3 minutes, a tack-free time of 18 minutes and complete through-curing to glass (sealant applied in thickness of 9 mm to a glass plate) after 7 days. In addition, the sealant had a pleasant odour and good adhesion to glass, aluminium, PVC, sheet metal, steel, lacquered wood, varnished wood, polyamide and Al/Mg alloy. Even after the sealant had been stored at 60 C. for 4 weeks, the sealant was stable and colourless.

Example 6

(24) A silicone rubber mixture of the following composition was produced by mixing the components under reduced pressure:

(25) TABLE-US-00006 550 g alpha,omega-dihydroxyl-terminated polydimethylsiloxane having viscosity of 80 000 cSt 264 g polydimethylsiloxane having viscosity of 100 cSt 81 g curing agent: P(SEHE)3 1 g aminoalkyltrialkoxysilane 91 g untreated fumed silica 0.2 g catalyst: alkyltin carboxylate 9 g adhesion promoter: dimethylaminopropyltrimethoxysilane

(26) The resulting sealant was transparent and colourless and had a skin time of 8 minutes, a tack-free time of 17 minutes and complete through-curing to glass (sealant applied in thickness of 9 mm to a glass plate) after 7 days. In addition, the sealant had a pleasant odour and good adhesion to glass, aluminium, PVC, sheet metal, steel, wood, lacquered wood, varnished wood, polyamide and Al/Mg alloy. Even after the sealant had been stored at 60 C. for 4 weeks, the sealant was stable and colourless.

Example 7

(27) A silicone rubber mixture of the following composition was produced by mixing the components under reduced pressure:

(28) TABLE-US-00007 504 g alpha,omega-dihydroxyl-terminated polydimethylsiloxane having viscosity of 80 000 cSt 328 g polydimethylsiloxane having viscosity of 100 cSt 65 g curing agent: P(ES)3 1 g aminoalkyltrialkoxysilane 85 g untreated fumed silica 1 g catalyst: 1:1 (w/w) mixture of dialkyltin oxide and tetraalkoxy- silane 16 g adhesion promoter: 1:1 (w/w) mixture of dialkylaminoalkyltri- alkoxysilane and tris(trialkoxysilylalkyl) isocyanurate

(29) The resulting sealant was transparent and colourless and had a skin time of 13 minutes, a tack-free time of 32 minutes and complete through-curing to glass (sealant applied in thickness of 9 mm to a glass plate) after 6 days. Elongation at break was determined as 340%. In addition, the sealant had a pleasant odour and good adhesion to glass, aluminium, PVC, sheet metal, steel and wood. Even after the sealant had been stored at 60 C. for 4 weeks, the sealant was stable and colourless.

(30) Thus, the curing agent of the invention leads to sealants having good properties. More particularly, the curing agent leads to a sealant which does not cause any odour nuisance on deployment, has good elongation at break and, even in the course of prolonged storage at elevated temperature, remains stable and does not become discoloured.

Example 8 (Curing Agent Comprising a Mixture of Silanes)

(31) A silicone rubber mixture of the following composition was produced by mixing the components under reduced pressure:

(32) TABLE-US-00008 550 g alpha,omega-dihydroxyl-terminated polydimethylsiloxane having viscosity of 80 000 cSt 268 g polydimethylsiloxane having viscosity of 100 cSt 40 g curing agent 1: tris(2-ethylhexylsalicylato)methylsilane 40 g curing agent 2: tris(2-ethylhexylsalicylato)propylsilane 1 g aminoalkyltrialkoxysilane 85 g untreated fumed silica 0.7 g catalyst: 1:1 (w/w) mixture of dialkyltin oxide and tetraalkoxy- silane 16 g adhesion promoter: 1:3 (w/w) mixture of dialkylaminoalkyltri- alkoxysilane and tris(trialkoxysilylalkyl)dialkylenediurea

(33) The resulting sealant was transparent and colourless and had a skin time of 15 minutes, a tack-free time of 60 minutes and complete through-curing to glass (sealant applied in thickness of 9 mm to a glass plate) after 6 days. Elongation at break was determined as 370%. In addition, the sealant had a pleasant odour and good adhesion to glass, aluminium, PVC, sheet metal, steel and wood. Even after the sealant had been stored at 60 C. for 4 weeks, the sealant was stable and colourless.

(34) Thus, curing agents of the invention comprising a mixture of silanes lead to sealants having good properties. More particularly, curing agents of the invention comprising a mixture of silanes lead to a sealant which does not cause any odour nuisance on deployment, has good elongation at break and, even in the course of prolonged storage at elevated temperature, remains stable and does not become discoloured.

Example 9 (Curing Agent Comprising a Mixture of Silanes)

(35) A silicone rubber mixture of the following composition was produced by mixing the components under reduced pressure:

(36) TABLE-US-00009 550 g alpha,omega-dihydroxyl-terminated polydimethylsiloxane having viscosity of 80 000 cSt 268 g polydimethylsiloxane having viscosity of 100 cSt 40 g curing agent 1: tris(2-ethylhexylsalicylato)methylsilane 40 g curing agent 2: tris(2-ethylhexylsalicylato)propylsilane 1 g aminoalkyltrialkoxysilane 85 g untreated fumed silica 0.7 g catalyst: 1:1 (w/w) mixture of dialkyltin oxide and tetraalkoxy- silane 16 g adhesion promoter: 1:3 (w/w) mixture of dialkylaminoalkyltri- alkoxysilane and tris(trialkoxysilylalkyl) isocyanurate

(37) The resulting sealant was transparent and colourless and had a skin time of 16 minutes, a tack-free time of 38 minutes and complete through-curing to glass (sealant applied in thickness of 9 mm to a glass plate) after 7 days. Elongation at break was determined as 310%. In addition, the sealant had a pleasant odour and good adhesion to glass, aluminium, PVC, sheet metal, steel and wood. Even after the sealant had been stored at 60 C. for 4 weeks, the sealant was stable and colourless.

Example 10 (Curing Agent Comprising a Mixture of Silanes)

(38) A silicone rubber mixture of the following composition was produced by mixing the components under reduced pressure:

(39) TABLE-US-00010 540 g alpha,omega-dihydroxyl-terminated polydimethylsiloxane having viscosity of 80 000 cSt 310 g polydimethylsiloxane having viscosity of 100 cSt 28 g curing agent 1: methyltris(diethyllactamido)silane 20 g curing agent 2: vinyltris(diethyllactamido)silane 1 g aminoalkyltrialkoxysilane 96 g untreated fumed silica 1.2 g catalyst: 1:1 (w/w) mixture of dialkyltin oxide and tetraalkoxy- silane 10 g adhesion promoter: dialkylaminoalkyltrialkoxysilane

(40) The resulting sealant was transparent and colourless and had a skin time of 10 minutes, a tack-free time of 50 minutes and complete through-curing to glass (sealant applied in thickness of 9 mm to a glass plate) after 7 days. In addition, the sealant was odourless and had good adhesion to glass, aluminium, PVC, sheet metal, steel, wood, lacquered wood, varnished wood, polyamide and Al/Mg alloy. Even after the sealant had been stored at 60 C. for 4 weeks, the sealant was stable and colourless.

(41) Thus, curing agents of the invention comprising a mixture of silanes lead to sealants having good properties. More particularly, curing agents of the invention comprising a mixture of silanes lead to a sealant which does not cause any odour nuisance on deployment, has good elongation at break and, even in the course of prolonged storage at elevated temperature, remains stable and does not become discoloured.

(42) Thus, the curing agents of the invention are of good suitability as curing agent in silicone rubber compositions (sealants).